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Abstract We describe the development of a new three‐dimensional off‐line chemical transport model (CTM). The CTM has been produced by combining the existing, and closely related, TOMCAT and SLIMCAT models. The new CTM (TOMCAT/SLIMCAT) has a flexible vertical coordinate which can use both σ‐ p and σ‐θ levels. A novel approach is used for the σ‐θ coordinate to ensure a smooth transition between levels. The CTM has different options for calculating vertical transport in the stratosphere, depending on the coordinate chosen. The CTM also has different options for other processes such as advection scheme, radiation scheme and meteorological forcing. We have used different configurations of the new CTM to perform tests on stratospheric tracer transport. Using ECMWF ERA‐40 analyses, the σ‐ p coordinate model gives a stratospheric age of air which is much too low, and a tropical tape‐recorder signal which propagates vertically too rapidly. Changing the coordinates to σ‐θ levels, and still using the analyses to calculate the vertical motion, removes spurious vertical mixing and improves the modelled age of air significantly, although it still tends to underestimate the observations. If we use a radiation scheme to calculate the stratospheric diabatic transport in the σ‐θ model we get a greater age of air, and the best overall agreement with the observations of age of air from in situ data and estimates of the tape‐recorder signal from Halogen Occultation Experiment CH 4 and H 2 O data. Based on the model results, interannual variability can cause age of air changes of up to ∼1 year in the mid/high‐latitude lower stratosphere. Similar differences can be caused by changing to winds from the UK Met Office. For the year 2001, where two sets of ECMWF analyses are available, the operational analyses produce a greater stratospheric age of air than the ERA‐40 re‐analyses. Copyright © 2006 Royal Meteorological Society.
Martyn P. Chipperfield (Sat,) studied this question.